1. Field of the Invention
This invention relates to a display apparatus for displaying an optical image transmitted via a cable taking the form of an optical fiber bundle.
2. Description of the Prior Art
It is known to use a cable comprising an optical fiber bundle, obtained by bundling a plurality of optical fibers, to transmit an optical image from one end of the bundle to the other by projecting the optical image with the aid of a lens, for example, on the front end face of the bundle and allowing the image to be transferred through the bundle and formed in the rear end face of the bundle.
For successful transmission of the optical image, however, the optical fiber bundle is required to be such that, at the opposite ends thereof, the individual optical fibers thereof are disposed geometrically in mutually corresponding positions. Otherwise, no faithful transmission of the optical image is obtained because the optical image in the front end face of the bundle and the optical image in the rear end face do not coincide with each other.
In the production of an optical fiber bundle of great length, the practice of imparting flexibility to the bundle, for example, by intertwisting individual optical fibers or strands of optical fibers so as to facilitate the handling of the bundle when the bundle is wound on a drum has found widespread acceptance. When the individual optical fibers are intertwisted or otherwise manipulated, however, there inevitably ensues the consequence that the geometric positions of the individual optical fibers relative to one another differ in the opposite end faces of the bundle. Such an arrangement of fibers, wherein the relative geometric positions of the fibers differ at the opposite end faces of a fiber bundle, will hereinafter be referred to as a "random" arrangement of fibers. Moreover, when the number of optical fibers making up an optical fiber bundle is increased in an attempt to improve the definition of an optical image to be transmitted, it often happens that optical fibers having flaws, such as chippings and fractures which are responsible for blots appearing in the displayed image, may possibly mingle with other flawless optical fibers in the bundle.
In conventional optical image transmission, therefore, because of the foregoing problems only a rather slender optical fiber bundle of small length is customarily used for medical observation of internal organs such as the stomach and the esophagus as a fiberscope (such as a gastroscope).
There is eager demand for application of the optical image transmission technique to remote monitoring of important phenomena occurring at places such as the interiors of nuclear reactors and blast furnaces which are hardly accessible by human beings. This demand is not easily met at present, in actuality, because manufacture of a long optical fiber bundle having as many optical fibers as desired and yet warranting perfect mutual conformity of the geometric positions of individual optical fibers in the opposite end faces of the bundle is extremely difficult for the reason given above.